Paint Profiler: erase bitmap before replaying SkPicture

Source/platform/graphics/GraphicsContextRecorder.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 86 matching lines @@
         return nullptr;
     return adoptRef(new GraphicsContextSnapshot(picture));
 }
 
 PassOwnPtr<Vector<char> > GraphicsContextSnapshot::replay(unsigned fromStep, unsigned toStep, double scale) const
 {
     int width = ceil(scale * m_picture->width());
     int height = ceil(scale * m_picture->height());
     SkBitmap bitmap;
     bitmap.allocPixels(SkImageInfo::MakeN32Premul(width, height));
+    bitmap.eraseARGB(0, 0, 0, 0);
     {
         ReplayingCanvas canvas(bitmap, fromStep, toStep);
         canvas.scale(scale, scale);
         canvas.resetStepCount();
         m_picture->draw(&canvas, &canvas);
     }
     OwnPtr<Vector<char> > base64Data = adoptPtr(new Vector<char>());
     Vector<char> encodedImage;
     if (!PNGImageEncoder::encode(bitmap, reinterpret_cast<Vector<unsigned char>*>(&encodedImage)))
         return nullptr;
@@ skipping 26 matching lines @@
 }
 
 PassRefPtr<JSONArray> GraphicsContextSnapshot::snapshotCommandLog() const
 {
     LoggingCanvas canvas(m_picture->width(), m_picture->height());
     m_picture->draw(&canvas);
     return canvas.log();
 }
 
 } // namespace blink